Low energy curing offset and letterpress printing inks and printing process

ABSTRACT

The present invention relates to the field of low energy radically curable inks for offset or letterpress printing of security documents. In particular, the invention relates to low energy radically curable offset or letterpress printing inks for offset or letterpress printing on a substrate or security document, said low energy radically curable inks having a viscosity in the range of about 2.5 to about 25 Pa s at 40° C. and 1000 s −1  and comprising radically curable (meth)acrylate compounds, one or more one or more photoinitiators of formula (I), one or machine readable materials and one or more fillers and/or extenders.

FIELD OF THE INVENTION

The present invention relates to the field of the protection of securitydocument against counterfeit and illegal reproduction. The presentinvention relates to the field of low energy radically curable offsetand letterpress printing inks and processes for producing securityfeatures on security documents.

BACKGROUND OF THE INVENTION

With the constantly improving quality of color photocopies and printingsand in an attempt to protect security documents such as banknotes, valuedocuments or cards, transportation tickets or cards, tax banderols, andproduct labels that have no reproducible effects against counterfeiting,falsifying or illegal reproduction, it has been the conventionalpractice to incorporate various security means in these documents.Typical examples of security means include security threads, windows,fibers, planchettes, foils, decals, holograms, watermarks, security inkscomprising optically variable pigments, magnetic or magnetizablethin-film interference pigments, interference-coated particles,thermochromic pigments, photochromic pigments, luminescent,infrared-absorbing, ultraviolet-absorbing or magnetic compounds.

Machine readable inks, such as for example magnetic inks, luminescentinks and IR absorbing inks, have been widely used in the field ofsecurity documents, in particular for banknotes printing, to confer thesecurity document an additional covert security feature. The protectionof security document against counterfeit and illegal reproductionprovided by covert security features relies on the concept that suchfeatures typically require specialized equipment and knowledge for theirdetection. In the field of security and protecting value documents andvalue commercial goods against counterfeiting, falsifying and illegalreproduction, it is known in the art to apply machine readable securityinks by different printing processes including printing processes usinghigh viscous or pasty inks such as offset printing, letterpress printingand intaglio printing (also referred in the art as engraved steel die orcopper plate printing). UV radically curable pigmented offset printinginks and UV radically curable pigmented letterpress printing inks areused in the field of the protection of security document againstcounterfeit and illegal reproduction to be applied as thin layers onsecurity documents in the form of security features.

Failure of the ink to rapidly and efficiently dry results in set off.Set off occurs when a printing ink which is not dried or cured adheresto the back of a printed substrate placed on top of it during thestacking of printed substrates as it comes off the presses (see e.g.U.S. Pat. No. 4,604,952). This is a particular problem during theprinting of security features on security documents, especiallybanknotes, since said documents typically carry a multitude ofoverlapping or partially overlapping security features which are appliedone after the other. If the previously applied security feature, e.g. abackground image or graphic pattern, has not yet sufficiently dried, thewhole multi-step printing process is not only delayed but also theso-obtained security feature may still suffer from set off or markingdue to set off on the machine during any subsequent printing or processoperations.

Conventional banknote printing processes use printing technologiesincluding offset printing, intaglio printing, silkscreen printing,flexography printing and letterpress printing, in a series of stepsseparated by drying periods of the just printed ink layer.

During a conventional banknote printing process, offset inks are appliedduring one of the first steps of overall multi-step printing process,wherein the offset printing is followed with an intaglio printing step.If a security feature is printed on a security document such as abanknote by an offset printing process and if said security featuresuffer from low surface curing properties, the subsequent intaglioprinting step might be delayed or produce set off. During an intaglio(also referred in the art as engraved copper plate printing and engravedsteel die printing) printing process, an engraved steel cylindercarrying a heated plate engraved with a pattern or image to be printedis supplied with inks of inking cylinder(s) (or chablon cylinder), eachinking cylinder being inked in at least one corresponding color to formsecurity features. The inked intaglio plate is brought into contact withthe substrate and the ink is transferred under pressure from theengravings of the intaglio printing plate onto the substrate to beprinted forming a thick printing layer in the form of reliefs, saidresulting from the ink layer thickness and the embossing of thesubstrate. One of the distinguishing features of the intaglio printingprocess is that the thickness of the ink transferred to the substratecan be varied from a few micrometers to several tens of micrometers byusing correspondingly shallow or respectively deep recesses of theintaglio printing plate. Accordingly, it is critical that the offsetinks are completely dried before starting intaglio printing in order toavoid any set off issues, said set off issues being pronounced for thesubsequent intaglio printing process due the high pressure applied topiles of printed substrates.

During a conventional banknote printing process, letterpress inks areapplied during one of the last steps of overall multi-step printingprocess, wherein the letterpress printing is followed with a cuttingprocess using for example a trimmer or guillotine wherein sheetscomprising a plurality of banknotes are cut so as to form individualbanknotes for their circulation. If a security feature is printed on asecurity document such as a banknote by a letterpress printing processand if said security feature suffer from low surface curing properties,the subsequent cutting step might be delayed or produce set off.Letterpress printing (also referred in the art as letterpress reliefprinting and typography), is a method consisting of transferring an inkfrom a hard metal printing plate comprising raised elements, such asletters, numbers, symbols, lines or dots. The raised printing elementsare coated with a layer of ink of constant thickness by the applicationof rollers. The ink is then transferred to an article or a substrate.The letterpress printing technique is typically used for the purpose ofnumbering the banknotes, i.e. providing banknotes with one or moreunique serial numbers. Accordingly, it is critical that the letterpressinks are completely dried before starting the cutting step in order toavoid any set off issues, said set off issues being pronounced for thesubsequent cutting step process due the high pressure applied to pilesof printed substrates.

UV radically curable inks are cured by free radical mechanismsconsisting of the activation by energy of one or more free radicalphotoinitiators which liberate free radicals which in turn initiate thepolymerization so as to form a layer or coating. Known free radicalphotoinitiators include acetophenones, benzophenones,alpha-aminoketones, alpha-hydroxyketones, phosphine oxides and phosphineoxide derivatives and benzyldimethyl ketals.

With the aim of providing environmentally friendly solutions, systemsand UV mercury medium pressure lamps for curing inks with low energy (LEor HUV) have been developed. Low energy mercury medium pressure lampshave emission spectrum in UV-A and UV-B regions and have less than 5% ofthe UV energy in UV-C emission. Said systems and lamps produce a reducedamount of light emitted in the ozone-generating wavelengths of thespectrum that occurs with conventional UV lamps such as medium pressuremercury lamps.

UV-curing efficiency of a coating or ink layer depends not only on theoverlap of the emission spectrum of the irradiation source used for saidcuring and the absorption spectrum of the photoinitiator comprised inthe coating or ink layer but also on the intensity of the emissionspectrum of the irradiation source and on the molar absorptioncoefficient of the photoinitiator at the wavelength of the emissionspectrum of the irradiation source. Accordingly, UV-curing of coatingsor ink layers comprising conventionally used free radicalphotoinitiators with UV-LE lamps suffers from a reduced curingefficiency as a result of the poor overlap of the emission spectrum ofUV-LE lamp with the absorption of the said conventionally used freeradical photoinitiators, thus leading to slow or poor curing or curingdefects.

Since machine readable security inks typically comprise a high amount ofpigments, said inks are particularly difficult to cure due to thefiltering effect of said pigments which reduced the amount ofUV-irradiation available for the photoinitiators.

With the aim of overcoming the poor curing properties of UV-curing withUV-LE lamps, acyl-phosphine photoinitiators have been used due to theirred-shifted absorption spectrum. However, acyl-phosphine photoinitiatorsare known to be particularly sensitive to oxygen inhibition and nothighly efficient for surface curing or curing of thin layers. Oxygeninhibition during UV-curing of coatings layers is in particular an issuefor thin UV-curable layers.

Thus, there remains a need for low energy curable pigmented offset andletterpress printing inks, and processes for printing security featureson security documents at high speed (i.e. industrial speed), saidprinting security features combining good surface cure and goodthrough-cure properties after having cured said inks with a radiationwavelength of 280 to 400 nm (low energy lamps).

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to overcome thedeficiencies of the prior art as discussed above. This is achieved bythe provision of a using one or more photoinitiators of formula (I) in aradically curable offset or letterpress printing ink, wherein saidradically curable offset or letterpress printing ink has a viscosity inthe range of about 2.5 to about 25 Pa s at 40° C. and 1000 s⁻¹ andcomprises radically curable (meth)acrylate compounds, one or machinereadable materials selected from the group consisting of luminescentmaterials, magnetic materials, IR absorbing materials and mixturesthereof, and one or more filers and/or extenders.

Described herein are low energy radically curable offset or letterpressprinting inks having a viscosity in the range of about 2.5 to about 25Pa s at 40° C. and 1000 s⁻¹ for printing a security feature on asubstrate or security document, said low energy radically curable offsetor letterpress printing ink comprising:

i) from about 10 wt. % to about 80 wt. % of radically curable(meth)acrylate compounds;

ii) from about 1 wt. % to about 20 wt. % of one or more photoinitiatorsof formula (I):

-   -   wherein    -   R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹, R¹⁰, R¹¹, R¹², R¹³, R¹⁴ are        identical or different from each other and are selected from the        group consisting of hydrogen, C₁-C₄-alkyls (e.g. methyl, ethyl,        propyl, isopropyl, cyclopropyl, butyl, isobutyl, sec-butyl,        tert-butyl, cyclobutyl) and halogens, in particular fluorine,        chlorine and bromine;        iii) from about 1 wt. % to about 60 wt % of one or machine        readable materials selected from the group consisting of        luminescent materials, magnetic materials. IR absorbing        materials and mixtures thereof; and        iv) from about 0.5 wt. % to about 20 wt. % of one or more filers        and/or extenders,        the weight percents being based on the total weight of the low        energy radically curable offset or letterpress printing ink.

Described herein are processes for producing for printing a securityfeature on a substrate by an offset or letterpress printing processcomprising the steps of applying the low energy radically curable offsetor letterpress printing ink described herein by offset printing orletterpress printing so as to form a coating or layer, and curing thecoating or layer with a UV lamp (280 to 400 nm) at a dose of at least 50mJ/cm², preferably at least 100 mJ/cm².

Also described herein are security features comprising a layer orcoating made of the low energy radically curable offset or letterpressprinting ink described herein. Described herein are uses of the securityfeatures described herein for the protection of a security documentagainst counterfeiting or fraud and security documents comprising one ormore of the security features described herein.

Also described herein are security documents comprising the one or moresecurity features described herein.

Also described herein are uses of the one or more photoinitiatorsdescribed herein in an amount from about 1 wt. % to about 20 wt. % forproducing a low energy radically curable offset or letterpress printingink having a viscosity in the range of about 2.5 to about 25 Pa s at 40°C. and 1000 s⁻¹, said low energy radically curable offset or letterpressprinting ink being suitable for printing a security feature on asecurity document, said low energy radically curable offset orletterpress printing ink comprising

i) from about 10 wt. % to about 80 wt. % of radically curable(meth)acrylate compounds;

ii) from about 1 wt. % to about 60 wt. % of one or machine readablematerials selected from the group consisting of luminescent materials,magnetic materials, IR absorbing materials and mixtures thereof; and

iii) from about 0.5 wt. % to about 20 wt. % of one or more fillersand/or extenders, the weight percents being based on the total weight ofthe low energy radically curable offset or letterpress printing ink.

DETAILED DESCRIPTION Definitions

The following definitions clarify the meaning of the terms used in thedescription and in the claims.

As used herein, the indefinite article “a” indicates one as well as morethan one and does not necessarily limit its referent noun to thesingular.

As used herein, the term “about” means that the amount, value or limitin question may be the specific value designated or some other value inits neighborhood. Generally, the term “about” denoting a certain valueis intended to denote a range within ±5% of the value. For example, thephrase “about 100” denotes a range of 100±5, i.e. the range from 95 to105. Generally, when the term “about” is used, it can be expected thatsimilar results or effects according to the invention can be obtainedwithin a range of ±5% of the indicated value. However, a specificamount, value or limit supplemented with the term “about” is intendedherein to disclose as well the very amount, value or limit as such, i.e.without the “about” supplement.

As used herein, the term “and/or” means that either all or only one ofthe elements of said group may be present. For example, “A and/or B”shall mean “only A, or only B, or both A and B”. In the case of “onlyA”, the term also covers the possibility that B is absent, i.e. “only A,but not B”.

As used herein, the term “one or more” means one, two, three, four, etc.

The term “comprising” as used herein is intended to be non-exclusive andopen-ended. Thus, for instance an ink solution comprising a compound Amay include other compounds besides A. However, the term “comprising”also covers, as a particular embodiment thereof, the more restrictivemeanings of “consisting essentially of” and “consisting of”, so that forinstance “an ink solution comprising a compound A” may also(essentially) consist of the compound A.

Where the present description refers to “preferred”embodiments/features, combinations of these “preferred”embodiments/features shall also be deemed as disclosed as long as thiscombination of “preferred” embodiments/features is technicallymeaningful.

The term “security feature” is used to denote an image, pattern orgraphic element that can be used for authentication purposes.

The term “security document” refers to a document which is usuallyprotected against counterfeit or fraud by at least one security feature.Examples of security documents include without limitation valuedocuments and value commercial goods.

The descriptions of specific embodiments of the present invention arepresented for purposes of illustration and description. They are notintended to be exhaustive or to limit the present invention to theprecise forms disclosed, and obviously many modifications and variationsare possible in light of the above teaching. The exemplary embodimentswere chosen and described in order to best explain the principles of thepresent invention and its practical application, to thereby enableothers skilled in the art to best use the present invention and variousembodiments with various modifications as are suited to the particularuse contemplated.

The present invention provides low energy radically curable offset orletterpress printing inks for producing (printing) a security feature ona security document by an offset printing process or by a letterpressprinting process. The present invention further provides securityfeatures comprising a coating or a layer made of the low energyradically curable offset or letterpress printing ink described hereinand security documents comprising the one or more security featuresdescribed herein.

The low energy radically curable offset or letterpress printing inkdescribed herein has a viscosity in the range of about 2.5 to about 25Pa s at 40° C. and 1000 s⁻¹; the viscosities being measured on a HaakeRoto-Visco RV1 with a cone 2 cm 0.5°.

The low energy radically curable offset or letterpress printing inkdescribed herein comprises radically curable (meth)acrylate compounds.The radically curable (meth)acrylate compounds described herein arepresent in an amount from about 10 wt. % to about 80 wt. %, preferablyfrom about 20 wt. % to about 80 wt. %, the weight percents being basedon the total weight of the low energy radically curable offset orletterpress printing ink described herein.

Radically curable compounds are cured by free radical mechanismsconsisting of the activation by energy of one or more photoinitiatorswhich liberate free radicals which in turn initiate the polymerizationso as to form a layer or coating.

The radically curable (meth)acrylate compounds described preferablyconsist of one or more radically curable (meth)acrylate oligomers andone or more radically curable (meth)acrylate monomers. The term“(meth)acrylate” in the context of the present invention refers to theacrylate as well as the corresponding methacrylate. The radicallycurable (meth)acrylate oligomers described herein are preferablyselected from the group consisting of polyepoxy (meth)acrylates,(meth)acrylated oils, (meth)acrylated epoxidized oils, polyester(meth)acrylates, polyether (meth)acrylates, aliphatic or aromaticpolyurethane (meth)acrylates, silicone (meth)acrylates, polyamino(meth)acrylates, polyacrylic acid (meth)acrylates, polyacrylate esters(meth)acrylates and mixtures thereof, more preferably selected from thegroup consisting of polyepoxy (meth)acrylates, polyester(meth)acrylates, aliphatic or aromatic polyurethane (meth)acrylates,silicone (meth)acrylates, polyamino (meth)acrylates, polyacrylic acid(meth)acrylates, polyacrylate esters (meth)acrylates and mixturesthereof. The radically curable (meth)acrylate monomers described hereinare preferably selected from the group consisting of2(2-ethoxyethoxy)ethyl (meth)acrylate, 2-phenoxyethyl (meth)acrylate,C12/C14 alkyl (meth)acrylate, C16/C18 alkyl (meth)acrylate, caprolactone(meth)acrylate, cyclic trimethylolpropane formal (meth)acrylate,nonylphenol (meth)acrylate, isobornyl (meth)acrylate, isodecyl(meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate,octyldecyl (meth)acrylate, tridecyl (meth)acrylate, methoxypoly(ethylene glycol) (meth)acrylate, polypropylene glycol(meth)acrylate, tetrahydrofurfuryl (meth)acrylate, 1,3-butylene glycoldi(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanedioldi(meth)acrylate, 3-methyl-1,5-pentanedioldi(meth)acrylate, alkoxylateddi(meth)acrylate, esterdiol di(meth)acrylate, bisphenol Adi(meth)acrylate, bisphenol A ethoxylate di(meth)acrylate, bisphenol Adiglycidyl ether di(meth)acrylate, ethylene glycol di(meth)acrylate,diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate,tetraethylene glycol di(meth)acrylate, dipropyleneglycoldi(meth)acrylate, tripropylene glycol di(meth)acrylate, polyethyleneglycol di(meth)acrylate, neopentyl glycol di(meth)acrylate,tricyclodecane dimethanol di(meth)acrylate, trimethyloipropanetri(meth)acrylate, glyceryl tri(meth)acrylate, propoxylatedtrimethylolpropane tri(meth)acrylate, propoxylated glyceryltri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritoltetra(meth)acrylate, propoxylated pentaerythritol tri(meth)acrylate,trimethylolpropane tri(meth)acrylate, tris (2-hydroxy ethyl)isocyanurate tri(meth)acrylate, ditrimethylolpropanetetra(meth)acrylate, trimethylolpropane tri(meth)acrylate,dipentaerythritol penta(meth)acrylate, pentaerythritoltetra(meth)acrylate, dipentaerythritol tetra(meth)acrylate,dipentaerythritol penta(meth)acrylate, dipentaerythritolhexa(meth)acrylate and their ethoxylated equivalents as well as mixturesthereof, more preferably from the group consisting of 2-phenoxyethyl(meth)acrylate, isodecyl (meth)acrylate, 1,4-butanedioldi(meth)acrylate, 1,6-hexanediol di(meth)acrylate, diethylene glycoldi(meth)acrylate, triethylene glycol di(meth)acrylate, dipropyleneglycoldi(meth)acrylate, tripropylene glycol di(meth)acrylate, polyethyleneglycol di(meth)acrylate, neopentyl glycol di(meth)acrylate,trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylateand their ethoxylated equivalents as well as mixtures thereof, and stillmore preferably from the group consisting of trimethylolpropanetriacrylate (TMPTA), pentaerytritol triacrylate (PTA),tripropyleneglycol diacrylate (TPGDA), dipropyleneglycol diacrylate(DPGDA), 1,6-hexanediol diacrylate (HDDA) as well as mixtures thereof.

The low energy radically curable offset or letterpress printing inkdescribed herein comprising the (meth)acrylate compounds describedherein may further comprise one or more vinyl ethers and/or theirethoxylated equivalents. Suitable vinyl ethers may be selected from thegroup consisting of ethyl vinyl ether (EVE), n-butyl vinyl ether (NBVE),iso-butyl vinyl ether (IBVE), cyclohexyl vinyl ether (CHVE),2-ethylhexyl vinyl ether (EHVE), 1,4-butanediol divinyl ether (BDDVE),diethyleneglycol divinyl ether (DVE-2), triethyleneglycol divinyl ether(DVE-3), 1,4-cyclohexanedimethanol divinyl ether (CHDM-di), hydroxybutylvinyl ether (HBVE), 1,4-cyclohexanedimethanol mono vinyl ether(CHDM-mono).

The low energy radically curable offset or letterpress printing inkdescribed herein described herein comprises the one or morephotoinitiators of formula (I):

whereinR¹, R². R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹, R¹⁰, R¹¹, R¹², R¹³, R¹⁴ areidentical or different from each other and are selected from the groupconsisting of hydrogen, C₁-C₄-alkyls (e.g. methyl, ethyl, propyl,isopropyl, cyclopropyl, butyl, isobutyl, sec-butyl, tert-butyl,cyclobutyl) and halogens, in particular fluorine, chlorine and bromine.

According to a preferred embodiment, the low energy radically curableoffset or letterpress printing ink described herein comprises the one ormore photoinitiators of formula (I), wherein R¹, R², R³, R⁴, R⁵, R⁶, R⁷,R⁸, R⁹, R¹⁰, R¹¹, R¹², R¹³, R¹⁴ are identical or different from eachother and are selected from the group consisting of hydrogen, andC₁-C₄-alkyls (e.g. methyl, ethyl, propyl, isopropyl, cyclopropyl, butyl,isobutyl, sec-butyl, tert-butyl, cyclobutyl) and halogens, in particularfluorine, chlorine and bromine.

According to a preferred embodiment, the low energy radically curableoffset or letterpress printing ink described herein comprises the one ormore photoinitiators of formula (I), wherein R¹, R², R³, R⁴, R⁵, R⁶, R⁷,R⁸, R⁹, R¹⁰, R¹¹, R¹², R³, R¹⁴ are hydrogens. In other words, the one ormore photoinitiators is of formula (II):

It should further be appreciated that the invention also extends tocompounds in which one or more of the atoms have been replaced by anisotopic variant, such as for example one or more hydrogen atoms may bereplaced by ²H or ³H and/or one or more carbon atoms may be replaced by¹⁴C or ¹³C.

Compounds of structure (II) suitable as photoinitiators described hereinare commercially available as Genocure PBZ from Rahn Switzerland (CASNumber 2128-93-0).

The one or more photoinitiators comprised in the low energy radicallycurable offset or letterpress printing ink described herein arepreferably present in a total amount from about 1 wt. % to about 20wt-%, more preferably about 1 wt. % to about 15 wt. %, the weightpercents being based on the total weight of the low energy radicallycurable offset or letterpress printing ink described herein.

The low energy radically curable offset or letterpress printing inkdescribed herein may further comprise one or more inert resins (i.e.resins which do not take part in the polymerization reaction). Inertresins may be used to adjust the viscosity of the radically curableoffset or letterpress printing ink described herein, to lower the glasstransition temperature of an ink layer prepared with the radicallycurable offset or letterpress printing ink described herein, or toincrease the adhesion of an ink layer prepared with the radicallycurable offset or letterpress printing ink described herein The one ormore inert resins are preferably selected from the group consisting ofhydrocarbons (such as e.g. styrene based hydrocarbon resins), acrylics(such as e.g. acrylic co-polymers), styreneallylalcohols, phenolicresins, rosin-modified resins, ketonic resins, alkyd resins and mixturesthereof. When present, the one or more inert resins described herein arepresent in the low energy curable wet offset or letterpress printing inkdescribed herein in an amount from about 0.1 wt. % to about 10 wt-%,preferably in an amount from about 0.5 wt. % to about 2 wt. %, theweight percents being based on the total weight of the low energycurable wet offset or letterpress printing ink described herein.

The low energy radically curable offset or letterpress printing inkdescribed herein may further comprise one or more sensitizers inconjunction with the one or more photoinitiators in order to achieveefficient curing. Typical examples of suitable sensitizers includewithout limitation isopropyl-thioxanthone (ITX),1-chloro-2-propoxy-thioxanthone (CPTX), 2-chioro-thioxanthone (CTX),2-methoxythioxanthone (MeOTX) and 2,4-dlethyl-thoxanthone (DETX),polymeric derivatives of those and mixtures thereof. When present, theone or more sensitizers described herein are present are present in thelow energy radically curable offset or letterpress printing ink in anamount from about 0.1 wt. % to about 5 wt-%, preferably in an amountfrom about 0.5 wt. % to about 2 wt-%, the weight percents being based onthe total weight of the low energy radically curable offset orletterpress printing ink described herein.

The low energy radically curable offset or letterpress printing inkdescribed herein may further comprise one or more UV-stabilizers inorder to stabilize said ink in particular during its storage. Typicalexamples of suitable UV-stabilizers include without limitation,hydroquinone, hydroquinone monomethyl ether, 4-t-butylcatechol,4-t-butyl-phenol, 2,6-di-t-butyl-4-methyl-phenol (BHT), pyrogallol,phenothiazine (PTZ), 2,4-diazabicyclo[2.2.2] octane (DABCO), copper (II)salts (such as e.g. copper (II) phenoxide, copper (II) acetylacetonate,copper (II) gluconate, copper (II) tartrate, copper (II) acetate, copper(II) carbamate, copper (II) thiocarbamate, copper (II) dithiocarbamateor copper (II) dimethyl dithiocarbamate), copper (I) salts (such as e.g.copper (I) chloride or copper (I) acetate),tris[N-(hydroxyl-κO)—N-(nitroso-κO)benzenaminato]-aluminum and mixturesthereof. When present, the one or more UV-stabilizers described hereinare present in the low energy radically curable offset or letterpressprinting ink in an amount from about 0.1 wt. % to about 5 wt-%,preferably in an amount from about 0.5 wt. % to about 2 wt-%, the weightpercents being based on the total weight of the low energy radicallycurable offset or letterpress printing ink described herein.

The low energy radically curable offset or letterpress printing inkdescribed herein further comprises one or more machine readablematerials selected from the group consisting of luminescent materials,magnetic materials, IR absorbing materials and mixtures thereof. As usedherein, the term “machine readable material” refers to a material whichexhibits at least one distinctive property which is detectable by adevice or a machine, such as for example a magnetic detector (when themachine readable materials have magnetic properties) or an IR-camera(when the machine readable materials have IR-absorbing properties), andwhich can be comprised in a security feature made from the low energyradically curable offset or letterpress printing ink described herein soas to confer a way to authenticate said security feature by the use of aparticular equipment for its detection and/or authentication. The one ormore machine readable materials described herein are present in anamount from about 1 wt. % to about 60 wt. %, preferably from about 5 wt.% to about 40 wt. %, the weight percents being based on the total weightof the low energy radically curable offset or letterpress printing ink.

Typical example of luminescent materials include without limitationinorganic pigments (inorganic host crystals or glasses doped withluminescent ions), organic or organometallic (complexes of luminescention(s) with organic ligand(s)) substances). Luminescent compounds canabsorb certain types of energy acting upon them and subsequently emit atleast partially this absorbed energy as electromagnetic radiation.Luminescent compounds are detected by exposing with a certain wavelengthof light and analyzing the emitted light. Down-converting luminescentcompounds absorb electromagnetic radiation at a higher frequency(shorter wavelength) and at least partially re-emit it at a lowerfrequency (longer wavelength). Up-converting luminescent compoundsabsorb electromagnetic radiation at a lower frequency and at leastpartially re-emit part of it at a higher frequency. Light emission ofluminescent materials arises from excited states in atoms or molecules.The radiative decay of such excited states has a characteristic decaytime, which depends on the material and can range from 10⁻⁹ seconds upto various hours. Both fluorescent and phosphorescent compounds aresuitable for present invention. In the case of phosphorescent compounds,measurement of decay characteristics may also be carried out and used asa machine-readable feature. Luminescent compounds in pigment form havebeen widely used in inks (see U.S. Pat. No. 6,565,770, WO 2008/033059 A2and WO 2008/092522 A1). Examples of luminescent compounds include amongothers sulphides, oxysulphides, phosphates, vanadates, etc. ofnon-luminescent cations, doped with at least one luminescent cationchosen from the group consisting of transition-metal and the rare-earthions; rare earth oxysulfides and rare-earth metal complexes such asthose described in WO 2009/005733 A2 or in U.S. Pat. No. 7,108,742.Examples of inorganic compounds materials include without limitationLa₂O₂S:Eu, ZnSiO₄:Mn, and YVO₄:Nd. When present, the one or moreluminescent materials are preferably present in an amount from about 1to about 30 wt. %, the weight percents being based on the total weightof the low energy radically curable offset or letterpress printing ink.

Magnetic materials exhibit particular, detectable magnetic properties ofthe ferromagnetic or ferrimagnetic type and include permanent magneticmaterials (hard-magnetic materials with coercivity Hc>1000 A/m) andmagnetizable materials (soft-magnetic materials with coercivity Hc⇐1000A/m according to IEC60404-1 (2000)). Typical examples of magneticmaterials include iron, nickel, cobalt, manganese and their magneticalloys, carbonyl iron, chromium dioxide CrO₂, magnetic iron oxides (e.g.Fe₂O₃; Fe₃O₄), magnetic ferrites M(II)Fe(III)₂O₄ and hexaferritesM(II)Fe(III)₁₂O₁₉, the magnetic garnets M(III)₃Fe(III)₅O₁₂ (such asYttrium iron garnet Y₃Fe₅O₁₂) and their magnetic isostructuralsubstitution products and particles with permanent magnetization (e.g.CoFe₂O₄). Magnetic pigments particles comprising a magnetic corematerial which is surrounded (coated) by at least one layer of anothermaterial such as those described in WO 2010/115986 A2 may also be usedfor the present invention. When present, the one or more magneticmaterials are preferably present in an amount from about 5 to about 60wt. %, the weight percents being based on the total weight of the lowenergy radically curable offset or letterpress printing ink.

Infrared (IR) absorbing materials, i.e. materials absorbing in thenear-infrared (NIR) range of the electromagnetic spectrum, mostgenerally in the 700 nm to 2500 nm wavelength range, are widely knownand used as marking materials in security applications to confer to theprinted documents an additional, covert, security element which helptheir authentication. For example, security features having IR-absorbingproperties have been implemented in banknotes for use by automaticcurrency processing equipment, in banking and vending applications(automatic teller machines, automatic vending machines, etc.), in orderto recognize a determined currency bill and to verify its authenticity,in particular to discriminate it from replicas made by color copiers. IRabsorbing materials include IR absorbing inorganic materials, glassescomprising substantial amounts of IR-absorbing atoms or ions or entitieswhich display IR-absorption as a cooperative effect, IR absorbingorganic materials and IR absorbing organometallic materials (complexesof cation(s) with organic ligand(s), wherein either the separate cationand/or the separate ligand, or both in conjunction, have IR-absorbingproperties). Typical examples of IR absorbing materials include amongothers carbon black, quinone-diimmonium or aminium salts, polymethines(e.g. cyanines, squaraines, croconaines), phthalocyanine ornaphthalocyanine type (IR-absorbing pi-system), dithiolenes,quaterrylene diimides, metal (e.g. transition metals or lanthanides)phosphates, lanthanum hexaboride, indium tin oxide, antimony tin oxidein nano-particulate form and doped tin(IV) oxide (cooperative propertyof the SnO₄ crystal). IR absorbing materials comprising a transitionelement compound and whose infrared absorption is a consequence ofelectronic transitions within the d-shell of transition element atoms orions such as those described in WO 2007/060133 A2 may also be used forthe present invention. When present, the one or more IR absorbingmaterials are preferably present in an amount from about 1 to about 40wt. %, the weight percents being based on the total weight of the lowenergy radically curable offset or letterpress printing ink.

The low energy radically curable offset or letterpress printing inkdescribed herein further comprises one or more filers and/or extendersin an amount from about 0.5 to about 20 wt. %, preferably from about 1to about 10 wt. %, the weight percents being based on the total weightof the low energy radically curable offset or letterpress printing ink.Preferably the one or more fillers and/or extenders are elected from thegroup consisting of carbon fibers, talcs, micas (muscovites),wolastonites, day (calcinated days and china days), kaolins, carbonates(e.g. calcium carbonate, sodium aluminum carbonate), slicates (e.g.magnesium silicate, aluminum silicate), sulfates (e.g. magnesiumsulfate, barium sulphate), titanates (e.g. potassium titanate), aluminahydrates, silica (also including fumed silicas), montmorillonites,graphites, anatases, rutiles, bentonites, vermiculites, zinc whites,zinc sulphides, wood flours, quartz flours, natural fibers, syntheticfibers and combinations thereof. More preferably, the one or morefillers and/or extenders are selected from the group consisting ofcarbonates (e.g. calcium carbonate, sodium aluminum carbonate), silicas,talcs, clays and mixtures thereof.

The low energy radically curable offset or letterpress printing inkdescribed herein may further comprise a) one or more dyes, and/or b)inorganic pigments, organic pigments or mixtures thereof. Dyes suitablefor inks are known in the art and are preferably selected from the groupcomprising reactive dyes, direct dyes, anionic dyes, cationic dyes, aciddyes, basic dyes, food dyes, metal-complex dyes, solvent dyes andmixtures thereof. Typical examples of suitable dyes include withoutlimitation coumarines, cyanines, oxazines, uranines, phtalocyanines,indolinocyanines, triphenylmethanes, naphtalocyanines,indonanaphtalo-metal dyes, anthraquinones, anthrapyridones, azo dyes,rhodamines, squarilium dyes, croconium dyes. Typical examples of dyessuitable for the present invention include without limitation C.I. AcidYellow 1, 3, 5, 7, 11, 17, 19, 23, 25, 29, 36, 38, 40, 42, 44, 49, 54,59, 61, 70, 72, 73, 75, 76, 78, 79, 98, 99, 110, 111, 121, 127, 131,135, 142, 157, 162, 164, 165, 194, 204, 236, 245; C.I. Direct Yellow 1,8, 11, 12, 24, 26, 27, 33, 39, 44, 50, 58, 85, 86, 87, 88, 89, 98, 106,107, 110, 132, 142, 144; C.I. Basic Yellow 13, 28, 65; C.I. ReactiveYellow 1, 2, 3, 4, 6, 7, 11, 12, 13, 14, 15, 16, 17, 18, 22, 23, 24, 25,26, 27, 37, 42; C.I. Food Yellow 3, 4; C.I. Acid Orange 1, 3, 7, 10, 20,76, 142, 144; C.I. Basic Orange 1, 2, 59; C.I. Food Orange 2; C.I.Orange B; C.I. Acid Red 1, 4, 6, 8, 9, 13, 14, 18, 26, 27, 32, 35, 37,42, 51, 52, 57, 73, 75, 77, 80, 82, 85, 87, 88, 89, 92, 94, 97, 106,111, 114, 115, 117, 118, 119, 129, 130, 131, 133, 134, 138, 143, 145,154, 155, 158, 168, 180, 183, 184, 186, 194, 198, 209, 211, 215, 219,221, 249, 252, 254, 262, 265, 274, 282, 289, 303, 317, 320, 321, 322,357, 359; C.I. Basic Red 1, 2, 14, 28; C.I. Direct Red 1, 2, 4, 9, 11,13, 17, 20, 23, 24, 28, 31, 33, 37, 39, 44, 46, 62, 63, 75, 79, 80, 81,83, 84, 89, 95, 99, 113, 197, 201, 218, 220, 224, 225, 226, 227, 228,229, 230, 231, 253; C.I. Reactive Red 1, 2, 3, 4, 5, 6, 7, 8, 11, 12,13, 15, 16, 17, 19, 20, 21, 22, 23, 24, 28, 29, 31, 32, 33, 34, 35, 36,37, 38, 39, 40, 41, 42, 43, 45, 46, 49, 50, 58, 59, 63, 64, 108, 180;C.I. Food Red 1, 7, 9, 14; C.I. Acid Blue 1, 7, 9, 15, 20, 22, 23, 25,27, 29, 40, 41, 43, 45, 54, 59, 60, 62, 72, 74, 78, 80, 82, 83, 90, 92,93, 100, 102, 103, 104, 112, 113, 117, 120, 126, 127, 129, 130, 131,138, 140, 142, 143, 151, 154, 158, 161, 166, 167, 168, 170, 171, 182,183, 184, 187, 192, 193, 199, 203, 204, 205, 229, 234, 236, 249, 254,285; C.I. Basic Blue 1, 3, 5, 7, 8, 9, 11, 55, 81; C.I. Direct Blue 1,2, 6, 15, 22, 25, 41, 71, 76, 77, 78, 80, 86, 87, 90, 98, 106, 108, 120,123, 158, 160, 163, 165, 168, 192, 193, 194, 195, 196, 199, 200, 201,202, 203, 207, 225, 226, 236, 237, 246, 248, 249; C.I. Reactive Blue 1,2, 3, 4, 5, 7, 8, 9, 13, 14, 15, 17, 18, 19, 20, 21, 25, 26, 27, 28, 29,31, 32, 33, 34, 37, 38, 39, 40, 41, 43, 44, 46, 77; C.I. Food Blue 1, 2;C.I. Acid Green 1, 3, 5, 16, 26, 104: C.I. Basic Green 1, 4; C.I. FoodGreen 3; C.I. Acid Violet 9, 17, 90, 102, 121; C.I. Basic Violet 2, 3,10, 11, 21; C.I. Acid Brown 101, 103, 165, 266, 268, 355, 357, 365, 384;C.I. Basic Brown 1; C.I. Acid Black 1, 2, 7, 24, 26, 29, 31, 48, 50, 51,52, 58, 60, 62, 63, 64, 67, 72, 76, 77, 94, 107, 108, 109, 110, 112,115, 118, 119, 121, 122, 131, 132, 139, 140, 155, 156, 157, 158, 159,191, 194; C.I. Direct Black 17, 19, 22, 32, 39, 51, 56, 62, 71, 74, 77,94, 105, 106, 107, 108, 112, 113, 117, 118, 132, 133, 146, 154, 168;C.I. Reactive Black 1, 3, 4, 5, 6, 8, 9, 10, 12, 13, 14, 18, 31; C.I.Food Black 2; C.I. Solvent Yellow 19, C.I. Solvent Orange 45, C.I.Solvent Red 8, C.I. Solvent Green 7, C.I. Solvent Blue 7, C.I. SolventBlack 7; C.I. Disperse Yellow 3, C.I. Disperse Red 4, 60, C.I. DisperseBlue 3, and metal azo dyes disclosed in U.S. Pat. Nos. 5,074,914,5,997,622, 6,001,161, JP 02-080470, JP 62-190272, JP 63-218766. Suitabledyes for the present invention may be infrared absorbing dyes orluminescent dyes. When present, the one or more dyes used in the lowenergy radically curable offset or letterpress printing ink describedherein are preferably present in an amount from about 1 to about 30 wt.%, the weight percents being based on the total weight of the low energyradically curable offset or letterpress printing ink.

Typical examples of organic and inorganic pigments include withoutlimitation C.I. Pigment Yellow 12, C.I. Pigment Yellow 42, C.I. PigmentYellow 93. C.I. Pigment Yellow 110, C.I. Pigment Yellow 147, C.I.Pigment Yellow 173, C.I. Pigment Orange 34. C.I. Pigment Orange 48, C.I.Pigment Orange 49, C.I. Pigment Orange 61, C.I. Pigment Orange 71, C.I.Pigment Orange 73. C.I. Pigment Red 9, C.I. Pigment Red 22, C.I. PigmentRed 23, C.I. Pigment Red 67, C.I. Pigment Red 122, C.I. Pigment Red 144,C.I. Pigment Red 146, C.I. Pigment Red 170, C.I. Pigment Red 177, C.I.Pigment Red 179, C.I. Pigment Red 185, C.I. Pigment Red 202, C.I.Pigment Red 224, C.I. Pigment Brown 6, C.I. Pigment Brown 7, C.I.Pigment Red 242, C.I. Pigment Red 254, C.I. Pigment Red 264, C.I.Pigment Brown 23, C.I. Pigment Blue 15, C.I. Pigment Blue 15:3, C.I.Pigment Blue 60, C.I. Pigment Violet 19, C.I. Pigment Violet 23, C.I.Pigment Violet 32, C.I. Pigment Violet 37, C.I. Pigment Green 7, C.I.Pigment Green 36, C.I. Pigment Black 7, C.I. Pigment Black 11, C.I.Pigment White 4, C.I. Pigment White 6. C.I. Pigment White 7. C.I.Pigment White 21, C.I. Pigment White 22, antimony yellow, lead chromate,lead chromate sulfate, lead molybdate, ultramarine blue, cobalt blue,manganese blue, chrome oxide green, hydrated chrome oxide green, cobaltgreen, cerium sulfide, cadmium sulfide, cadmium sulfoselenides, zincferrite, bismuth vanadate, Prussian blue, mixed metal oxides, azo,azomethine, methine, anthraquinone, phthalocyanine, perinone, perylene,diketopyrrolopyrrole, thioindigo, thiazinindigo, dioxazine,iminoisoindoline, iminoisoindolinone, quinacridone, flavanthrone,indanthrone, anthrapyrimidine and quinophthalone pigments. When present,the inorganic pigments, organic pigments or mixtures thereof describedherein are preferably present in an amount from about 0.1 to about 45wt. %, the weight percents being based on the total weight of the lowenergy radically curable offset or letterpress printing ink.

The low energy radically curable offset or letterpress printing inkdescribed herein may further comprise one or more waxes preferablyselected from the group consisting of synthetic waxes, petroleum waxesand natural waxes. Preferably the one or more waxes are selected fromthe group consisting of amide waxes, erucamide waxes, paraffin waxes,polyethylene waxes, polypropylene waxes, fluorocarbon waxes,polytetrafluoroethylene waxes, Fischer-Tropsch waxes, silicone fluids,bee waxes, candelilla waxes, montan waxes, camauba waxes and mixturesthereof, more preferably selected from the group consisting of paraffinwaxes, polyethylene waxes, fluorocarbon waxes, polytetrafluoroethylenewaxes carnauba waxes and mixtures thereof. When present, the one or morewaxes are preferably present in an amount from about 0.1 to about 5 wt.%, the weight percents being based on the total weight of the low energyradically curable offset or letterpress printing ink.

As known by those skilled in the art, the low energy radically curableoffset or letterpress printing ink described herein may further compriseone or more solvents and/or diluents.

The low energy radically curable offset or letterpress printing inkdescribed herein may further comprise additives that include, but arenot limited to, one or more of the following components as well ascombinations of these: co-initiators, anti-settling agents, anti-foamingagents, surfactants and other processing aids known in the field ofinks. Additives described herein may be present in the low energycurable wet offset printing inks described herein in amounts and informs known in the art, including in the form of so-callednano-materials where at least one of the dimensions of the particles isin the range of 1 to 1000 nm.

The low energy radically curable offset or letterpress printing inkdescribed herein is typically prepared by a method comprising a step ofdispersing, mixing and/or milling all the ingredients described herein,the one or more machine readable materials described herein, the one ormore one or more filers and/or extenders described herein, the one ormore dyes described herein when present, the inorganic pigments, organicpigments or mixtures thereof described herein when present, the one ormore waxes described herein when present, and the one or more additiveswhen present in the presence of the (meth)acrylate compounds describedherein, thus forming pasty compositions. The one or more photoinitiatorsdescribed herein may be added to the ink either during the dispersing ormixing step of all other ingredients or may be added at a later stage.

As described herein, the process described herein comprises a step ofapplying the low energy radically curable offset or letterpress printingink described herein by offset printing or letterpress printing so as toform a coating or layer, and curing the coating or layer with a UV lamp(280 to 400 nm) at a dose of at least 50 mJ/cm², preferably at least 100mJ/cm². As described hereafter, the dose may be measured using a UVPower Puck® II radiometer from EIT, Inc., U.S.A.

The coating or layer made of the low energy radically curable offset orletterpress printing ink described herein is UV-cured with a UV LE lamppreferably selected from Baldwin UV Ltd. United Kingdom, IST METZ GmbHGermany or Dr. Hönle AG, Germany.

In order to gain a better distinction of the tested photoinitiators'reactivity and to investigate the kinetic of the reaction as a functionof the dose, the low energy radically curable offset or letterpressprinting ink described herein was cured at two doses of 100 mJ/cm²(typical industrial dose) and 50 mJ/cm² with a UV-LE lamp by varying thetransportation belt speed.

The process described herein is particularly suitable for producing asecurity feature on a substrate that is suitable as substrate for asecurity document. According to one preferred embodiment, the securityfeature is used as background printing on the substrate to besubsequently printed or processed. This means that on top of thesecurity feature printed by the process described herein, i.e. theimage, pattern or graphic element that serves for authenticationpurposes, further security features or non-security features are printedor applied in one or more further printing or applying runs and thesecurity feature printed by the process described herein and the furthersecurity or non-security features overlap.

Typical examples of substrate include without limitation fiber-basedsubstrates, preferably substrates based on cellulosic fibers such aspaper, paper-containing materials, polymer-based substrates, compositematerials (e.g. substrates obtained by the lamination of paper layersand polymer films), metals or metalized materials, glasses, ceramics andcombinations thereof. Typical examples of polymer-based substrates aresubstrates made of ethylene- or propylene-based homo- and copolymerssuch as polypropylene (PP) and polyethylene (PE), polycarbonate (PC),polyvinyl chloride (PVC) and polyethylene terephthalate (PET). Typicalexamples of composite materials include without limitation multilayerstructures (e.g. laminates) of at least one paper layer and at least onepolymer film, including polymers such as those described above, as wellas paper-like substrates based on mixtures of cellulosic fibers andsynthetic polymer fibers. In one preferred embodiment the securityfeatures is printed on a substrate selected from offset papers andfiduciary papers. Offset paper is manufactured from wood-pulp cellulosewith properties that make the paper suitable for offset printing,including dimensional stability, resistance to curling, high surfacestrength, a surface free from foreign particles and a high level ofresistance to moisture penetration. Typically the basis weight of offsetpaper is of 30 g/m² to 250 g/m², preferably of 50 g/m² to 150 g/m².

Fiduciary paper (also referred in the art as security paper) ismanufactured from lignin-free, cotton-pulp cellulose. Compared to offsetpapers, additional properties of fiduciary papers include enhancedmechanical resistance (especially resistance to tearing and wearing),resistance to soiling and treatment against degradation bymicro-organisms (bacteria, virus and fungi). The mechanical resistanceof fiduciary papers may be enhanced by the introduction into the paper(cotton-based) pulp of synthetic fibers, and the anti-soilingperformance may be improved by coating or printing an anti-soilpolymeric layer prior to printing or applying the security features ofthe banknote. Usually, the treatment with biocides is combined with theanti-soil treatment. Typically, the fiduciary paper has a basis weightof 50 to 150 g/m², preferably of 80 to 120 g/m².

Furthermore, the use of fiduciary paper instead of offset paper adds anadditional element of anti-counterfeiting protection, since fiduciarypaper is manufactured on special paper-making machines that are onlyavailable to manufacturers of security paper, and since the supply chainis protected such as to prevent the fiduciary paper from being divertedto counterfeiters.

The term “security document” refers to a document having a value such asto render it potentially liable to attempts at counterfeiting or ilegalreproduction and which is usually protected against counterfeit or fraudby one or more security features. Examples of security documents includewithout limitation value documents and value commercial goods. Typicalexample of value documents include without limitation banknotes, deeds,tickets, checks, vouchers, fiscal stamps and tax labels, agreements andthe like, identity documents such as passports, identity cards, visas,bank cards, credit cards, transactions cards, access documents, securitybadges, entrance tickets, transportation tickets or titles, and thelike.

The term “value commercial good” refers to packaging material, inparticular for pharmaceutical, cosmetics, electronics or food industrythat may comprise one or more security features in order to warrant thatthe content of the packaging is genuine, like for instance genuinedrugs. Example of these packaging material include without limitationlabels such as authentication brand labels, tax banderoles, tamperevidence labels and seals. The security document described herein mayfurther comprise one or more additional layers or coatings either belowor on top of the security feature described herein. Should the adhesionbetween the substrate and the security feature described herein beinsufficient, for example, due to the substrate material, a surfaceunevenness or a surface inhomogeneity, an additional layer, coating or aprimer between the substrate and the security feature might be appliedas known for those skilled in the art.

With the aim of further increasing the security level and the resistanceagainst counterfeiting and illegal reproduction of security documents,the substrate may contain watermarks, security threads, fibers,planchettes, luminescent compounds, windows, foils, decals, coatings andcombinations thereof.

The substrate described herein, on which the low energy radicallycurable offset or letterpress printing ink described herein is applied,may consist of an intrinsic part of a security document, oralternatively, the low energy radically curable offset or letterpressprinting ink described herein is applied onto an auxiliary substratesuch as for example a security thread, security stripe, a foil, a decalor a label and consequently transferred to a security document in aseparate step.

Also described herein are uses of the one or more photoinitiatorsdescribed herein for producing the low energy radically curable offsetor letterpress printing ink described herein, said low energy radicallycurable offset or letterpress printing ink being suitable for printing asecurity feature on a security document.

EXAMPLE

The present invention is now described in more details with reference tonon-limiting examples. The Examples below provide more detail for thepreparation of low energy radically curable printing inks and use of thephotoinitiators according to the invention and comparative data.

The irradiation doses were determined by using a Power Pucke II device.The irradiation source (LE mercury medium pressure lamp) was turned on.The Power Pucke II device was placed on the belt of the irradiationapparatus designed to receive the samples to be irradiated. The PowerPuck® was irradiated with the irradiation source at different belt'sspeed. The dose was obtained by summing the doses of the UVA, UVB andUVC measured of the Power Pucke II. The following values were obtainedwith a HUV lamp type J (Baldwin, Ozone free lamp J7804045): 50 mJ/cm² at100 m/min, 100 mJ/cm² at 50 m/min.

Influence of the Photoinitiator on the Whiteness of a Printed and CuredLayer Made of Radically Curable Test Varnishes

The radically curable whiteness test varnishes of Table 2 were preparedin order to assess the influence of the tested photoinitiators describedin Table 1 on the whiteness (expressed in degrees Berger) of the printedtest varnish layer after UV-curing.

TABLE 1 photoinitiators Photoinitiator Number Photoinitiator Name CASNumber Structure P1 GENOCURE PBZ (from Rahn) [2128-93-0]

P2 DAROCUR ® 1173 (IGM Resins, previously BASF) [7473-98-5]

P3 IRGACURE ® 2959 (IGM Resins, previously BASF) [106797-53-9]

P4 ESACURE ® ONE (IGM Resins) [135452-42-5]

P5 ESACURE ® KIP 150 (IGM Resins, previously Lamberti) —

P6 IRGACURE ® 184 (IGM Resins, previously BASF) [947-19-3]

P7 GENOCURE BDK (Rahn) [24650-42-8]

P8 GENOPOL AB-2 (Rahn) —

P9 OMNIPOL 910 (IGM Resins) [886463-10-1]

P10 GENOCURE TPO (Rahn) [75980-60-8]

P11 IRGACURE ® 819 (IGM Resins, previously BASF) [162881-26-7]

P12 GENOCURE LTM (Rahn) —

P13 GENOCURE ITX (Rahn) [5495-84-1]

P14 OMNIPOL TX (IGM Resins) [813452-37-8]

P15 GENOPOL TX-2 — unknown (Rahn)

TABLE 2 radically curable transparent whiteness test varnishes(whiteness assessment) Ingredients wt. % Trimethylolpropane triacrylate(TMPTA) 63.05 (UV Monomer) (Allnex) Laropal ® 81 (Resin) (BASF) 33.95Photoinitiator of Table 1 (Photoinitiator) 3Preparation of the Radically Curable Whiteness Test Varnishes of Table 2

The radically curable transparent whiteness test varnishes wereindependently prepared by mixing the ingredients described in Table 2,except the photoinitiator with a mixer IKA T ULTRA TURRAX until thetemperature of the mixture reached 60-65° C.

The photoinitiators were independently added and the resulting mixtureswere independently dispersed with a Speedmixer DAC 150 for threeminutes, ground on a Loher mill (3×50 turns with a weight of 7.5 kg) anddispersed again with a Speedmixer DAC 150 for three minutes.

Printing and Curina Method

The radically curable transparent whiteness test varnishes wereindependently printed on a substrate (fiduciary paper, PapierfabrikLouisenthal GmbH) on a PrOfbau at 800 N, so as to obtain a printedlayer. The radically curable transparent whiteness test varnishes wereindependently cured and dried with a Hg-lamp (Aktiprint Mini 18.2) at abelt speed of 10 ml/min so as to form printed and cured test varnishlayers. The exact amount of the printed and cured test varnish layerswas calculated for each sample by weighting the substrate before andafter printing. The weight of the printed and cured test varnish layersof all the samples was 2 g/m²±3%.

The substrates carrying the printed and cured test varnish layer werekept in the dark for five days and the whiteness of said samples wasindependently and subsequently measured with a DC45 spectrophotometer:the whiteness value of the substrate (average value of threemeasurements) was subtracted from the whiteness value of the printed andcured test varnish layer (average value of three measurements). Largernegative values indicated a stronger yellowing of the printed and curedink layers. The whiteness of each sample is provided in Table 4.

Influence of the Photoinitiator on the Curing Efficiency of a Layer Madeof Radically Curable Black Fluorescent Offset Printing Inks

The radically curable black fluorescent offset printing inks of Table 3were prepared in order to assess the influence of the photoinitiators ofTable 1 on the curing of said inks. The curing performance of eachphotoinitiator was assessed by a counter-pressure test.

TABLE 3 radically curable black fluorescent offset printing inksIngredients wt. % EBECRYL ® 811 (polyester acrylate, oligomer) (Allnex)38.5 EBECRYL ® 1606 (bisphenol 25.41 A epoxy diacrylate diluted with20-25% of trimethylolpropane triacrylate monomer) (Allnex) EBECRYL ® 150(ethoxylated bisphenol A diacrylate oligomer) 11.55 (Allnex) MIRAMERM4004 3 (Pentaerythritol ethoxylated tetraacrylate monomer) (Rahn)FLORSTAB UV 1 (copper (II) dimethyl-dithiocarbamate, UV 1.54 stabilizer)(Kromatech) SHAMROCK WAX S394 N1 (Polyethylene wax) (Shamrock) 0.5FINNTALC M03 (Talc (Mg-Silicate), filler) (Grolman) 0.5 BENTONE 34(organic derivative of a bentonite clay, filler) 0.5 (Elementis)AEROSIL ® 200 (fumed silica, extender,) (Evonik) 0.5 Lumilux ® Yellow CD382 (salicylic acid derivative, luminescent 3 pigment) (HoneywellSpecialty Materials) Lumilux ® Green CD 394 (chinazolinon derivative,luminescent 3 pigment) (Honeywell Specialty Materials) Carbon Black 4A(black pigment, IR-absorbing pigment) (Orion) 7 Genocure EHA(2-ethylhexyl-4-dimethylaminobenzoate, 2 Co-initiator I) (Rahn)Photoinitiator of Table 1 3Preparation of the Radically Curable Black Fluorescent Offset PrintingInks of Table 3

The radically curable black fluorescent offset printing inks wereindependently prepared by mixing with the SpeedMixer™ (DAC 150 SP CM31from Hauschild Engineering) at room temperature the ingredients listedin Table 3, except the photoinitiators. The resulting pastes wereindependently ground on a SDY300 three roll mill in three passes (afirst pass at a pressure of 5 bars, a second and a third pass at apressure of 11 bars).

The photoinitiators were independently added to the pastes obtained asdescribed hereabove and the so-obtained inks were mixed in a SpeedMixer™(DAC 150 SP CM31 from Hauschid Engineering) at a speed of 2500 rpm forthree minutes at room temperature, ground on a Loher mil (3×50 turnswith a weight of 7.5 kg), and mixed again with the SpeedMixer™ for threeminutes.

The viscosity of the radically curable black fluorescent offset printinginks of Table 2 was measured at 40° C. and 1000 s⁻¹ on a HaakeRoto-Visco RV1 with a cone 2 cm 0.5°, linear speed increase 0-1000 sec⁻¹in 30 seconds and are provided in Table 4.

Printing and Curina Method

The radically curable black fluorescent offset printing inks wereindependently printed as a pattern (4.5 cm×23 cm) on a Guardian®substrate (Innovia) using a Profbau at a pressure of 1000 N (T=22° C.,relative humidity=54%). The patterns were divided in three parts andeach sample was dried under two different irradiation doses to assessthe curing performance of the photoinitiator using a HUV lamp type J(Baldwin, Ozone free lamp J7804045). The belt speed of the curing drierwas selected according to the different tested doses as measured with aUV Power Puck® II radiometer from EIT, Inc., U.S.A.: 100 m/min for adose of 50 mJ/cm², 50 m/min for a dose of 100 mJ/cm². The two doses wereselected such as to discriminate the different reactivity of the testedphotoinitiators and to investigate the kinetic of the reaction as afunction of the dose.

Curing Test by Counter-Pressure

For each sample, a drying test was carried out by forming an assemblyconsisting of a sheet of the substrate carrying the printed and curedink layer and a blank fiduciary paper placed on top of said substratecarrying the printed and cured ink layer and by submitting the so-formedassembly to a counter-pressure of 3.4 bars at 80° C. with an ORMAGIntaglio Proof Press. The substrate carrying the printed and cured inklayer and the blank fiduciary paper were separated and the opticaldensity of the blank fiduciary paper was checked for ink transfer.

The measured optical density Techkon SpectroDens Advanced, ISO 5-3status E, Techkon GmbH Germany) of the counterpressure test on thefiduciary blank paper after contact with the uncured ink layer on theGuardian® substrate is defined as 0% cure The measured optical densityof the blank fiduciary paper is defined as 100% cure (no setoff). Themeasured optical density of each sample at the different irradiationdoses are provided in Table 4.

Table 5 provides a summary of all the results.

TABLE 4 whiteness test results and curing tests results Δ Whiteness/(degree Berger) of the Viscosity/ radically (Pa · s) curable of theradically radically Counter-pressure curable curable test resultswhiteness black at two irradiation test fluorescent doses^([a])/(%)Photoinitiator varnishes inks 50 mJ/ 100 mJ/ Example Number of Table 2of Table 3 cm² cm² E1 P1 −1.70 3.40 80 85 C1 P2 −1.20 2.76 9 9 C2 P3−2.26 3.56 4 2 C3 P4 −2.19 3.99 45 60 C4 P5 −1.24 4.11 18 52 C5 P6 −1.063.13 2 4 C6 P7 −1.27 3.31 30 30 C7 P8 −2.30 3.81 7 7 C8 P9 −10.91 4.6383 78 C9 P10 −1.45 3.88 56 61 C10 P11 −1.67 3.90 68 77 C11 P12 −1.253.10 14 14 C12 P13 −4.31 3.33 62 66 C13 P14 −7.82 3.96 46 47 C14 P15−4.19 4.02 23 30 ^(a))Error margin: +/− 5%. ^(b))100 mJ/cm² correspondsto a typical industrial dose.

TABLE 5 summary of results Photoinitiator Counter- Example NumberPhotoinitiator Name Δ Whiteness pressure E1 P1 GENOCURE PBZ ++ ++ C1 P2DAROCUR ® 1173 ++ −− C2 P3 IRGACURE ® 2959 + −− C3 P4 ESACURE ® ONE + +C4 P5 ESACURE ® KIP 150 ++ + C5 P6 IRGACURE ® 184 ++ −− C6 P7 GENOCUREBDK ++ − C7 P8 GENOPOL AB-2 + −− C8 P9 OMNIPOL 910 −− ++ C9 P10 GENOCURETPO ++ + C10 P11 IRGACURE ® 819 ++ + C11 P12 GENOCURE LTM ++ −− C12 P13GENOCURE ITX − + C13 P14 OMNIPOL TX −− − C14 P15 GENOPOL TX-2 − −

As shown in Tables 4 and 5, the photoinitiator P1 (used in E1) led togood results not only in whiteness tests but also in counter-pressuretests even at very low irradiation doses (50 mJ/cm²). Whereas the inkscomprising the photoinitiators P9 (used in C8) and P11 (used in C12),respectively, led to good results in terms of curing performance at alow irradiation doses (at 50 mJ/cm²), an increase of the dose to 100mJ/cm² did not result in an increase of the curing performance to alevel comparable to E1. Moreover, the photoinitiator P9 (used in C8)exhibited a very poor performance in terms of whiteness. All thecomparative examples P2-P8 and P12-P15 resulted in either negativeyellowing performance (large negative values in whiteness test) and/orpoor curing performance.

The invention claimed is:
 1. A low energy radically curable offset orletterpress printing ink having a viscosity in the range of about 2.5 toabout 25 Pa s at 40° C. and 1000 s⁻¹ for printing a security feature ona substrate or security document, said low energy radically curableoffset or letterpress printing ink comprising: i) from about 10 wt. % toabout 80 wt. % of radically curable (meth)acrylate compounds; ii) fromabout 1 wt. % to about 20 wt. % of one or more photoinitiators offormula (I):

wherein R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹, R¹⁰, R¹¹, R¹², R¹³, R¹⁴ areidentical or different from each other and are selected from the groupconsisting of hydrogen, C₁-C₄-alkyls; iii) from about 1 wt. % to about60 wt. % of one or more machine readable materials selected from thegroup consisting of luminescent materials, magnetic materials, IRabsorbing materials and mixtures thereof; and iv) from about 0.5 wt. %to about 20 wt. % of one or more fillers and/or extenders, the weightpercents being based on the total weight of the low energy radicallycurable offset or letterpress printing ink.
 2. The low energy radicallycurable offset or letterpress printing ink according to claim 1, whereinat least one of the one or more photoinitiators is of formula (II):


3. The low energy radically curable offset or letterpress printing inkaccording to claim 1, wherein the one or more fillers and/or extendersare selected from the group consisting of carbonates, silicas, talcs,clays and mixtures thereof.
 4. The low energy radically curable offsetor letterpress printing ink according to claim 1, wherein the radicallycurable (meth)acrylate compounds consist of one or more radicallycurable (meth)acrylate oligomers and one or more radically curable(meth)acrylate monomers.
 5. The low energy radically curable offset orletterpress printing ink according to claim 1, further comprising a) oneor more dyes and/or b) inorganic pigments, organic pigments or mixturesthereof.
 6. The low energy radically curable offset or letterpressprinting ink according to claim 1, further comprising one or more waxesselected form the group consisting of paraffin waxes, polyethylenewaxes, fluorocarbon waxes, polytetrafluoroethylene waxes, carnauba waxesand mixtures thereof.
 7. A process for printing a security feature on asubstrate by an offset or letterpress printing process comprising thesteps of: a) applying the low energy radically curable offset orletterpress printing ink recited in claim 1 by offset printing orletterpress printing so as to form a coating or layer, and b) curing thecoating or layer with an ultraviolet lamp having a wavelength of 280 to400 nm at a dose of at least 50 mJ/cm².
 8. A security feature comprisinga coating or a layer made of the low energy radically curable offset orletterpress printing ink recited in claim
 1. 9. A security documentcomprising one or more security features recited in claim
 8. 10. The lowenergy radically curable offset or letterpress printing ink according toclaim 1, wherein R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹, R¹⁰, R¹¹, R¹², R¹³,R¹⁴ are selected from the group consisting of hydrogen, methyl, ethyl,propyl, isopropyl, cyclopropyl, butyl, isobutyl, sec-butyl, tert-butyl,cyclobutyl, fluorine, chlorine and bromine.
 11. The process for printinga security feature on a substrate by an offset or letterpress printingprocess according to claim 7, wherein in step b) curing the coating orlayer with the ultraviolet lamp is performed at a dose of at least 100mJ/cm².